Geotechnical Characterisation of Pumiceous Soils for Liquefaction Assessment

Abstract

Sands consisting of pumice particles are a common soil type found in several parts of the Waikato Basin and Bay of Plenty in the North Island of New Zealand. The pumice sands are distributed in the area due to powerful historic volcanic eruptions; with subsequent airborne transport and later erosion and river transport, they were mixed with other materials in the region. These pumice sands are highly crushable, compressible, and lightweight due to the vesicular nature of the particles, making engineering assessment of their properties problematic. When performing geotechnical assessments of these deposits, questions have been asked as to whether existing empirical correlations, derived primarily from normal (hard-grained) soils, would be applicable for the crushable pumiceous soils. Such a lack of guidance for the geotechnical characterisation and liquefaction assessment of pumiceous soils have long been highlighted by the geotechnical community. In this research, laboratory experiments, such as cyclic/monotonic triaxial tests and bender elements, were performed on high-quality undisturbed and reconstituted pumiceous samples taken from sites in Waikato Basin and Bay of Plenty to investigate the geotechnical behaviour of natural pumiceous (NP) soils. These were supplemented by various field tests, such as cone penetration tests (CPT) and shear wave velocity (Vs) profiling, which were conducted at the same sites where the undisturbed samples were collected. For comparison purposes, similar laboratory tests were also performed on hard-grained Toyoura sand. The laboratory-based results indicated considerably different behaviours of NP sands compared to hard-grained sands. For instance, NP sands showed lower small-strain shear modulus (Gmax) and Vs and higher liquefaction resistance as well as high deformability during cyclic loading when compared to hard-grained sands. Moreover, the field-based investigations using case studies from the 1987 Edgecumbe earthquake showed that the existing empirical CPT- and Vs- based methods developed for hard-grained sands significantly underestimated the liquefaction resistance of NP sands. Finally, results from both the laboratory studies and field characterisation were synthesised with consideration of the effect of various parameters such as degree of packing, overburden pressure, pumice contents, etc. Based on the results, attempts were made to develop design guidelines following Vs- and CPT-based approaches for the geotechnical characterisation and liquefaction evaluation of such problematic soils for use by the profession.